Sweep improvement by use of a static block between injection and production wells to delay cusp formation

ABSTRACT

A barrier comprising a slug of a fluid more viscous than formation hydrocarbons is injected via a well between the injection and production wells to retard the formation of the usual cusp at the interface between the injected driving and formation fluids as it advances toward a production well.

L 1 "J I -r r\ IN/"vi X1 1 ls iitii' pr iiif [72] Inventor George M. Wood [56] References Cited UNITED STATES PATENTS [31] 786,566 3 074 481 1 1963 Habermann 166/274X ii g 'f d 32* 3g: 3:109:487 11/1963 Hoyt 166/245 l z E 3,215,198 11 1965 Willman 166 263 g New York N Y 3,256,933 6/1966 Murphree et a1. 166/274X a cor Delaware 3,288,212 11/1966 OBrien et a1.... 166/274X p0 3,455,385 7/1969 Gogarty 166/274 Primary Examiner-Stephen .1. Novosad SWEEP IMPROVEMENT BY USE OF A STATIC A!t0rneysK. E. Kavanagh and Tohmas H. Whaley BLOCK BETWEEN INJECTION AND PRODUCTION WELLS TO DELAY CUSP FORMATION 13 Claims, 12 Drawing Figs. [52] US. Cl 166/245, ABSTRACT: A barrier comprising a slug of a fluid more 166/263,166/274 viscous than formation hydrocarbons is injected via a we [51] Int. Cl E2lb 43/16, between the injection and production wells to retard the for- E21b 43/20 mation of the usual cusp at the interface between the injected [50] Field of Search 166/245, driving and formation fluids as it advances toward a production well.

. PATENTEU JANZS I97! SHEET 1 OF 2 T1 1 (pm/,4)

SWEEP IMPROVEMENT BY USE OF A STATIC BLOCK BETWEEN INJECTION AND PRODUCTION WELLS TO DELAY CUSP FORMATION FIELD OF THE INVENTION This invention relates generally to the production of hydrocarbons from underground hydrocarbon-bearing formations, and more particularly, to a method for increasing the efficiency of the production of hydrocarbons therefrom.

DESCRIPTION OF THE PRIOR ART In the production of hydrocarbons from permeable underground hydrocarbon-bearing formations, it is customary to drill one or more boreholes or wells into the hydrocarbonbearing formation and produce hydrocarbons, such as oil, through designated production wells, either by the natural formation pressure or by pumping the wells. Sooner or later, the flow of hydrocarbons diminishes and/or ceases, even though substantial quantities of hydrocarbons are still present in the underground formations.

Thus, secondary recovery programs are now an essential part of the overall planning for virtually every oil and gas-condensate reservoir in underground hydrocarbon-bearing formations. In general, this involves injecting an extraneous fluid, such as water or gas, into the reservoir zone to drive formation fluids including hydrocarbons toward production wells by the process frequently referred to as floodingf Usually, this flooding is accomplished by injecting through wells drilled in a geometric pattern, the most common pattern being the fivespot.

When the driving fluid from the injection well reaches the production wells of a five-spot pattern, the areal sweep is about 71 percent. By continuing production considerably past breakthrough, it is possible to produce much of the remaining unswept portion. It would be a great economic benefit to be able to achieve a sweep of 100 percent of the hydrocarbonbearing formation. It would be an even greater benefit to be able to achieve it at breakthrough, so that it would not be necessary to produce large quantities of injected driving fluid.

It is understood that the failure of the driving flood in secondary recovery operations to contact or sweep all the hydrocarbon zone is due to the development of a cusp, at the interface between the driving and the driven fluids, which advances toward the production well. If other portions of the interface could be made to keep up, or if the cusp formation were delayed, e.g. by a block, a more complete zonal sweep would be possible. In the commonly assigned U.S. Pat. No. 3,393,735, issued to A. F. Altamira et al. on Jul. 23, 1968 for Interface Advance Control in Pattern Floods by Use of Control Wells, there is disclosed how an increased amount of hydrocarbons is produced and recovered from an underground hydrocarbon-bearing formation by employing at least three wells, penetrating such a formation, which wells are in line, to produce hydrocarbons from the formation via two of these wells including the middle well, as disclosed in the commonly assigned U.S. Pat. No. 3,109,487, issued to Donald L. Hoyt on Nov. 5, I963 for Petroleum Production by Secondary Recovery. In both these cited patents, there is disclosed how a production control well is positioned between the injection well and the production well and is kept on production after the injection fluid reaches it. In this manner, the cusp is pinned" down at the control well and while the zone swept out by the injection fluid before breakthrough at the outer production well is increased, there is an unwanted handling of considerable quantities of injection fluid at the control well.

Another aspect to increase the sweep is disclosed in the commonly assigned U.S. Pat. No. 3,393,734, issued to D. L. Hoyt et al. on July 23, 1968 and involves the retardation of the development of the cusp toward a production well. The method of achieving more uniform advance is to control the flow gradients so that the interface is spread out. This can be done either by choosing a particular geometry of well positions or by adjusting the relative production rates so that the velocity of advance is not predominantly in one direction. It can be done also by shifting the gradients frequently, in both direction and magnitude, thus preventing any one portion of an interface from advancing too far out of line.

SUMMARY OF THE INVENTION It is an overall object of the present invention to provide an improved secondary recovery procedure involving initially three wells in line as part of a well arrangement for exploiting a hydrocarbon-bearing formation, by changing the flow pat tern of the injected extraneous driving fluid, using for this purpose, strategically placed fluids having higher viscosity than the reservoir hydrocarbons and the extraneous driving fluid. The function of some of the wells in the arrangement may be changed at strategic times to gain maximum control of the interface or flood front.

A three-well group is arranged in line so that an end well is completed for production and the remaining two wells are offset with the outer well thereof being completed for injection. Secondary recovery by flooding may be initiated by injection of a highly viscous fluid via the inner of the offset wells with/ production from the end (outer) production well until such time as a sufficient volume ofthe highly viscous fluid has been injected to form a viscous bubble of desired size. Then the inner of the offset injection wells is shut in and injection of an extraneous driving fluid, such as water, is initiated at the outer injection well. The injected driving fluid is forced to travel a longer path around the viscous barrier and the formation of a cusp is blocked.

Flooding also may be initiated by simultaneous injection into the two offset wells, with the highly viscous fluid being put into the inner of these two wells, and the lower viscosity driving fluid being injected into the outer of the two offset wells, with the inner of the two wells being shut in after the desired volume of highly viscous fluid has been injected into the formation.

The flood can be initiated by injection of the extraneous driving fluid in the outer offset well and the placement of the viscous bubble could be delayed to any time until the interface reached the inner offset well, in which case the inner offset well could be used as a production well until interface breakthrough thereat, and then converted to an injection well for the placement of the viscous bubble. The well could be used to produce past breakthrough and pin'- the cusp until a more favorable interface is formed and then the viscous bubble could be injected.

Control of the sweep is accomplished by the relative viscosities of the high-viscosity bubble and the formation fluids to be displaced, the location of the bubble by the placement of the intermediate (inner offset) injection well, the size of the bubble, the shape of the bubble, and by the time of placement of the viscous bubble.

Any of a number of high-viscosity fluids can be used to create the viscous bubble, the primary consideration being that the fluid, after being emplaced, have a higher viscosity than the formation hydrocarbons which are to be swept, yet having a viscosity low enough to permit injection into the formation under reasonable circumstances. Some examples of this viscous fluid would be formation hydrocarbon fluids reacted with thickeners and water thickened with polymers to achieve the desired viscosities, hydrocarbons or crudes more viscous than the hydrocarbons in place in the formation, or any other fluids, such as molasses, which may be of the desired viscosity.

When the viscosity of the viscous bubble is not too much greater than the viscosity of the formation hydrocarbons, the viscous bubble moves toward the injection well and with proper combination of relative viscosities, size and placement of the bubble, the viscous fluid bubble can be moved to the production well and produced before, or shortly after, breakthrough of the driving fluid. This would be desirable in the case that a viscous hydrocarbon were used for the bubble.

Other objects, advantages and features of this invention will become apparent from a consideration of the specification with reference to the FIGS. of the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 discloses four units of an inverted five-spot pattern (prior art);

FIG. la is illustrative of the interface advance in the form of a cusp toward a comer production well in one quadrant of such a five-spot pattern undergoing secondary recovery (prior art);

FIG. 2 discloses one unit ofa nine-well diagonal pattern;

FIG. 2a, corresponding to FIG. 1a, illustrates the effect of a viscous bubble as it blocks and retards the formation of the cusp at the interface in its advance toward a corner production well in one quadrant of a nine-well diagonal pattern undergoing secondary recovery;

FIGS. 3a and 3b, illustrate in one quadrant of a nine-well diagonal pattern, the effect of using viscosity ratios which will permit the viscous bubble to migrate to the comer production well during a secondary recovery program;

FIGS. 40, 4b and 4c illustrate the changes in well functions in accordance with the movement of the interface during the sever phases of the production program in a l3-well pattern undergoing secondary recovery; and

FIGS. 5a, 5b and 5c correspond to FIGS. 4a, 4b and 4c during the production phases applied to a. l7-well pattern, undergoing secondary recovery.

The objects of the invention are achieved by the use of a viscous barrier to retard the formation of the usual cusp interface between the formation and injected fluids in its advance toward a production well of a selected well arrangement by the injection of a fluid more viscous than the formation hydrocarbons to form the barrier.

The specification and the FIGS. of the drawings schematically disclose and illustrate the practice and the advantages of the invention with well patterns and zonal sweep examples which are obtainable and have been observed both in secondary recovery operations and in potentiometric model studies which simulate secondary recovery operations. The model studies indicate a sweep obtained in an ideal reservoir, although the recovery from an actual sweep of a particular field may be greater or less, depending on field parameters.

Throughout the FIGS. of the drawings, the same symbols will be maintained as follows: P and P, represent respectively production wells at the corners and along the sides; I, and I, represent respectively the interior control wells and the central injection well of the arrangement; and, a solid circle indicates a production well, a crossed circle indicates shut-in well, an arrowed open circle indicates an injection well, and an open circle, a well site. Also clear areas are swept areas, right-diagonal sectioned areas indicate unswept in place formation fluids and left-diagonal sectioned areas the location of the highly viscous bubble. The diagonal x-x represents an axis of a production well and a pair of ofiset wells including an injection well.

Referring to FIG. 1, there is disclosed four units of an inverted five-spot pattern wherein the corner wells of each pattern unit are production wells, while the inner central well is I used for injection.

FIG. la illustrates the growth of the cusp in one quadrant of an inverted five-spot pattern unit, wherein the secondary flooding fluid is injected into the central well and production is maintained at the comer wells until breakthrough to result in a sweep of approximately 71 percent.

Referring to FIG. 2, there is disclosed a nine-well diagonal pattern, essentially the five-spot pattern with interior injection control wells positioned on the diagonals between the central injection well and the corner production wells,

FIG. 2a illustrates the improvement over prior art. The formation of a cusp is retarded or blocked by the placement in trol well, 1,. After bubble placement, this well is closed in and injection is started or continued at the central injection well, I In this FIG., the interior injection well, 1,, is located midway between the central injection well and the corner production well, P Using a volume of viscous fluid of about 13 percent of a quadrant of a pattern unit volume to form the bubble, the sweep of the formation fluids has been increased by 15 percent, over the sweep in a conventional inverted five-spot pattern as illustrated in FIG. la, for a total sweep of 86 percent. Another advantage of this invention is that sweeps can be obtained without any production of injected fluids.

In FIG. 3a, the-location of the interface of the driving fluid at the time of the injection of the highly viscous bubble via the interior injection well I,- is indicated by the dashed quadrant line, the original location of the bubble being indicated by the sectioned area within the dashed line. At a later stage in the migration of this bubble, the clear area in full outline is the swept area and the sectioned areas show the location of the bubble and the unswept area.

In FIG. 3b, the bubble has migrated to and has been produced to a large extend via the corner production well P, while the unswept area has been reduced further, indicating a more efficient sweep than in FIG. 2a.

FIG. 4a discloses the basic nine-spot pattern modified by the addition of four intermediate or interior injection wells, which can be positioned on the diagonals of the pattern for best advantage as indicated previously. It can be visualized also as a four-unit five-spot pattern, wherein the injection wells of the inverted five-spot pattern units are used for the injection of the viscous fluid, and the innermost production well of the four-unit five-spot pattern has been converted into an injection well for the driving fluid. With such a conversion, the positions of the intermediate wells have been predetermined and may not be situated for best effect.

The first phase of the production method requires injecting the highly viscous fluid via the four intermediate injection wells, I,, and production initiated and maintained at the remaining eight wells of the pattern until viscous bubbles of the desired size are produced at the four interior injection wells, at which time, these injection wells are shut in and injection of the driving fluid is initiated at the central injection well, while production is continued from the corner wells P and the side wells P, until breakthrough thereat as illustrated in FIG. 4b. Then as indicated in FIG. 40, the four side wells are closed in, production is continued at the comer production wells P, until breakthrough occurs.

FIG. 5a discloses a l7 -spot pattern which is formed by drilling a single injection well in a center of a 4 X 4 well square and converting the four surrounding wells to injection wells. In this l7-spot pattern, there are four corner production wells, two producing side wells on each side of the 4 X 4 well square, and four interior control wells for injection located on the diagonals of the pattern and positioned between the central injection well and the comer production wells.

Referring to FIGS. 5a, 5b and 50, there are illustrated three steps or phases of the production method as applied to the l7- spot pattern. 'In the first phase, illustrated in FIG. 5a, with injection maintained at the central injection well, injection 'of the viscous bubbles is initiated at the four interior injection wells, and production is maintained at the eight side wells and each of the comer wells until the desired size bubbles are achieved at the interior injection wells. In the second phase, as illustrated in FIG. 5b, while production is maintained at the comer production wells and the side wells, the four interior injection wells are shut in and injection maintained at the center well until breakthrough occurs at the side wells. Thereupon, the side wells P, are closed in, while production is maintained until breakthrough at the corner production wells, P and injection of the driving fluid is continued at the central well. Other combinations could be utilized such as production from the four interior wells until breakthrough and then placement of the viscous bubbles follows, after which the four wells are the formation of aviscous bubble via an interior injection conshut in.

It is obvious that this invention can be used to advantage with any pattern which has a well located in a position to place the viscous bubble to obstruct the cusping effect of an interface moving from an injection well to a production well.

Using either of the patterns in FIGS. 40 or 511, it would be possible to shape the bubble by injecting the viscous fluid at the l, wells and producing only from the P, wells. The viscous bubble would assume an elongated shape in the directions of the nearest production wells. When a bubble of sufficient size and desired shape has been formed, the l, wells would be shut in and injection of normal driving fluid started at the central injection well with production from all of the P, and P wells. Other combinations of production and injection wells could be used to obtain the desired bubble shape.

Any pattern, rate distribution, or other procedure such as placement of a viscous bubble, which retards the development, or the advance, of a cusp towards the production wells will increase the sweep of a field. Two principal means of doing this have been cited above, viz: (a) pinning down the cusp by locating production wells between the injection source and the outer production wells, and keeping such inner (or control) wells on production after breakthrough; and (b) spreading' out the cusp by pulling the front toward side wells until breakthrough thereat before allowing it to proceed toward the comer production wells of a pattern unit.

Herein has been disclosed still another method of delaying the advance of the interface in the form of a cusp toward an outer production well by injection via a control well of a slug of more viscous fluid than the reservoir fluids to serve as a static block.

Although emphasis has been placed in this disclosure on the practice of this invention as directed to a secondary recovery operation, particularly employing water or other similar aqueous fluid as the injection displacement fluid, the advantages obtainable in the practice of this invention are also realized in primary hydrocarbon production operations wherein the hydrocarbon-bearing formation is under the influence of either a water of gas drive, or both a water and a gas drive, and also in the instance of a secondary recovery operation wherein a gas, such as natural gas, is employed as the injection fluid. Moreover, the invention is applicable particularly to an arrangement of a pair of production wells in line with an injection well under the influence of an active water drive.

lclaim:

l. A method of producing formation fluids including hydrocarbons from an underground hydrocarbon-bearing formation which comprises penetrating said formation with at least three wells, a first well, a second well and a third well, said wells being substantially in line and the second and third wells being on one side of said first well with said second well closer thereto, injecting an extraneous driving fluid into said formation via said first well to displace fluids including hydrocarbons in said formation toward said second and third wells, producing said formation fluids including hydrocarbons from said formation via said second and third wells, ceasing producing said formation fluids and then injecting into said formation via said second well a fluid having a viscosity greater than that of the formation and driving fluids, maintaining producing said formation fluids including hydrocarbons from said formation via said third well while injecting extraneous fluid into said formation via said first well, the three wells in line being part of a nine-well diagonal pattern wherein the central well of said pattern is an injection well and the remaining pattern wells are production wells arranged in equal numbers on the diagonals of a quadrilateral, and producing said formation fluids via all of said production wells.

2. in a method as defined in claim 1, injecting the highly viscous fluid into said formation via said second well prior to breakthrough of said extraneous fluid thereinto.

3. in a method as defined in claim 1, injecting the highly viscous fluid into said formation via said second well upon breakthrough of said extraneous fluid thereinto.

4. In a method as defined in claim 1, said injecting of the highly viscous fluid via said second well being of a predetermined percentage of the pattern unit volume, to provide a static barrier between the first and third wells.

5. in a method of producing formation fluids as defined in claim 1, the injecting of said extraneous fluid into said formation via said first well continuing until breakthrough thereofat said third well.

6. in a method of producing formation fluids as defined in claim 1, concurrently initiating and maintaining producing said formation fluids from said second and third wells.

7. In a method of producing formation fluids as defined in claim 1, initiating producing formation fluids from said third well after injecting the highly viscous fluid into said formation via said second well.

8. In a method as defined in claim 1, the highly viscous fluid comprising formation hydrocarbon fluids treated with thickeners to increase the viscosity thereof prior to injecting into said formation via said second well, said extraneous driving fluid comprising natural gas.

9. A method of producing formation fluids including hydrocarbons from an underground hydrocarbon-bearing formation which comprises penetrating said formation with at least three wells, a first well, a second well and a third well, said wells being substantially in line and the second and third wells being on one side of said first well with said second well closer thereto, injecting an extraneous driving fluid into said formation via said first well to displace fluids including hydrocarbons in said formation toward said second and third wells, producing said formation fluids including hydrocarbons from said formation via said second and third wells, ceasing producing said formation fluids and then injecting into said formation via said second well a fluid having a viscosity greater than that of the formation and driving fluids, maintaining producing said formation fluids including hydrocarbons from said formation via said third well while injecting extraneous fluid into said formation via said first well, the three wells in line being part ofa l3-well pattern, wherein the central well of said pattern is an injection well and the remaining pattern wells are production wells arranged in equal numbers along the sides and on the diagonals of a quadrilateral, and producing said formation fluids via all of said production wells.

10. In a method of producing formation fluids as defined in claim 9, simultaneously initiating producing said formation fluids via all of said production wells.

11. In a method of producing formation as defined in claim 9, producing formation fluids via said wells located on the diagonals of said pattern adjacent said injection well and continuing producing therefrom until breakthrough of said extraneous fluid occurs, thereupon converting said aforementioned diagonal wells into injection wells and injecting into said formation via such converted wells the highly viscous fluid, and initiating and continuing producing from the side wells of said pattern until extraneous fluid breakthrough occurs thereat, thereupon initiating and maintaining producing said formation fluids via the corner wells of said pattern until extraneous fluid breakthrough occurs thereat.

12. A method of producing formation fluids including hydrocarbons from an underground hydrocarbon-bearing formation which comprises penetrating said formation with at least three wells, a first well, a second well and a third well, said wells being substantially in line and the second and third wells being on one side of said first well with said second well closer thereto, injecting an extraneous driving fluid into said formation via said first well to displace fluids including hydrocarbons in said formation toward said second and thirds wells, producing said formation fluids including hydrocarbons from said formation via said second and third wells, ceasing producing said. formation fluids and then injecting into said formation via said second well a fluid having a viscosity greater than that of the formation and driving fluids, maintaining producing said formation fluids including hydrocarbons from said formation viasaid third well while injecting extraneof the pattern until said extraneous fluid breakthrough occurs at individual production wells on said diagonals, thereupon converting said individual production wells into injection wells and injecting the highly viscous fluid into said formation via the converted injection wells, thereafter continuing producing said formation fluid via the remaining production wells until extraneous fluid breakthrough occurs thereat.

Po-wso UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Pat t N Dated January 26, 1971 Inventofls) George H. wood It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

' On the cover sheet: Col. 1, change the Assignee from "Texas Inc." to read "Texaco Inc.--; and in C01. 2, change the spelling of the attorneys to read Ho whaley-"'o I In the Specification:

Col. 3, line 25, change "sever" to --several--.

001. l, line 22, change "extend" to ---extent--.

001. 6, 11mm, change "thirds" to --third--.

Signed and sealed this 13th day of July 1971 (SEAL) Attest:

EDWARD M.FLETCHER,JR. WILLIAM E. SCHUYLER, JE Attesting Officer Commissioner of Patents 

2. In a method as defined in claim 1, injecting the highly viscous fluid into said formation via said second well prior to breakthrough of said extraneous fluid thereinto.
 3. In a method as defined in claim 1, injecting the highly viscous fluid into said formation via said second well upon breakthrough of said extraneous fluid thereinto.
 4. In a method as defined in claim 1, said injecting of the highly viscous fluid via said second well being of a predetermined percentage of the pattern unit volume, to provide a static barrier between the first and third wells.
 5. In a method of producing formation fluids as defined in claim 1, the injecting of said extraneous fluid into said formation via said first well continuing until breakthrough thereof at said third well.
 6. In a method of producing formation fluids as defined in claim 1, concurrently initiating and maintaining producing said formation fluids from said second and third wells.
 7. In a method of producing formation fluids as defined in claim 1, initiating producing formation fluids from said third well after injecting the highly viscous fluid into said formation via said second well.
 8. In a method as defined in claim 1, the highly viscous fluid comprising formation hydrocarbon fluids treated with thickeners to increase the viscosity thereof prior to injecting into said formation via said second well, said extraneous driving fluid comprising natural gas.
 9. A method of producing formation fluids including hydrocarbons from an underground hydrocarbon-bearing formation which comprises penetrating said formation with at least three wells, a first well, a second well and a third well, said wells being substantially in line and the second and third wells being on one side of said first well with said second well closer thereto, injecting an extraneous driving fluid into said formation via said first well to displace fluids including hydrocarbons in said formation toward said second and third wells, producing said formation fluids including hydrocarbons from said formation via said second and third wells, ceasing producing said formation fluids and then injecting into said formation via said second well a fluid havinG a viscosity greater than that of the formation and driving fluids, maintaining producing said formation fluids including hydrocarbons from said formation via said third well while injecting extraneous fluid into said formation via said first well, the three wells in line being part of a 13-well pattern, wherein the central well of said pattern is an injection well and the remaining pattern wells are production wells arranged in equal numbers along the sides and on the diagonals of a quadrilateral, and producing said formation fluids via all of said production wells.
 10. In a method of producing formation fluids as defined in claim 9, simultaneously initiating producing said formation fluids via all of said production wells.
 11. In a method of producing formation as defined in claim 9, producing formation fluids via said wells located on the diagonals of said pattern adjacent said injection well and continuing producing therefrom until breakthrough of said extraneous fluid occurs, thereupon converting said aforementioned diagonal wells into injection wells and injecting into said formation via such converted wells the highly viscous fluid, and initiating and continuing producing from the side wells of said pattern until extraneous fluid breakthrough occurs thereat, thereupon initiating and maintaining producing said formation fluids via the corner wells of said pattern until extraneous fluid breakthrough occurs thereat.
 12. A method of producing formation fluids including hydrocarbons from an underground hydrocarbon-bearing formation which comprises penetrating said formation with at least three wells, a first well, a second well and a third well, said wells being substantially in line and the second and third wells being on one side of said first well with said second well closer thereto, injecting an extraneous driving fluid into said formation via said first well to displace fluids including hydrocarbons in said formation toward said second and thirds wells, producing said formation fluids including hydrocarbons from said formation via said second and third wells, ceasing producing said formation fluids and then injecting into said formation via said second well a fluid having a viscosity greater than that of the formation and driving fluids, maintaining producing said formation fluids including hydrocarbons from said formation via said third well while injecting extraneous fluid into said formation via said first well, the three wells in line being part of a 17-well pattern, the central well being an injection well and the remaining wells being production wells located in equal numbers along the sides and on the diagonals of a quadrilateral.
 13. In a method of producing fluids as defined in claim 12, continuing injecting said extraneous fluid via said central well and producing simultaneously from all of the remaining wells of the pattern until said extraneous fluid breakthrough occurs at individual production wells on said diagonals, thereupon converting said individual production wells into injection wells and injecting the highly viscous fluid into said formation via the converted injection wells, thereafter continuing producing said formation fluid via the remaining production wells until extraneous fluid breakthrough occurs thereat. 